Star couplers, hubs, repeaters and the like are communications units whose significant object is to transmit the data signals over transmission systems like copper cables, glass fibers, radio-wave paths and the like, that is to receive such signals and to reforward them to connected final devices or further transmission systems. The data to be transmitted is not thereby changed. The data is usually in the form of data packets with a certain number of characters (bits). The number of bits which must be present at least in a data packet and the magnitude of the mechanism number is determined by the various standards, for example, the field bus standard EN 50170 (containing PROFIUS according to German Industrial Standard DIN 19245) for the different data protocols. Between each data packet and the next is a certain temporal gap in which nothing should be transmitted. The transmission systems here under consideration do not vary the data packets and are based upon the understanding that the data packets received lie within the specification of the given protocol and thus are error-free.
Apart from the data transmission, an error-free regeneration of the received data is an important object. Each data signal is influenced in various degrees and ways namely during the transmission. On the one hand, the signal amplitudes can be varied (for example by damping or pass frequency of the cable) while on the other hand however the position in time of a signal state alternation may vary (bit duration distortion, jitter and the like). The signal amplitudes are again regenerated by amplifiers and frequency components while for the correction of the point in time of the signal state change or timing so-called retimers are used.
In the state of the art, retimers are to be found which operate substantially in accordance with the following principle: Incoming data is sensed character for character and read into a memory. Simultaneously the bits are read out with the internally generated data clock bit for bit from the memory and transmitted. With large differences between the data clock of the incoming data packet and the internal clock or cadence, transmission errors can arise. If the internal clock is too fast, it seeks to transmit bits which have not been received. If the internal clock is too slow, the memory will fill. The memory is comparable to a water reservoir whose inflowing water quantity does not equal the. outflow. As the permissible cadence or clock difference and the maximum data packet length are known, the requisite memory size can be simply calculated. The greater the size of the memory required, the greater will also be the transit time since a part of the memory always must be initially filled with characters before transmission can commence because otherwise there is the danger that the memory will run empty. Thus each such retimer has the drawback that it produces in a disadvantageous manner a transit time delay of several bit periods which is not acceptable in more modern data communications and under many time-critical situations.